1. Field of the Invention
This invention relates generally to duplex reproduction apparatus and more particularly to the transfer of unfixed first and second images to first and second sides of a copy sheet before fixing of either of the images to the copy sheet.
2. Description of the Prior Art
Several techniques are known for forming duplex images on a final support medium such as a web or copy sheet. One such technique requires the use of two photoconductors upon which first and second images are formed. The support medium is generally passed between the photoconductors and the first and second images are transferred to opposite sides of the support medium. U.S. Pat. Nos. 3,548,783; 3,536,398; 3,580,670; 3,694,073; and 3,775,102 are exemplary of such a technique. The latter four patents disclose transferring both images to the support medium before fixing of either image. This technique is disadvantageous in increased cost, complexity and machine size and decreased reliability necessitated by the use of two photoconductive systems and two optical systems.
Another technique similar to the above but involving the use of only one photoconductor, utilizes an intermediate image transfer member to receive the first image formed on the photoconductor before transfer to a final support medium. The intermediate transfer member may be either a drum or roller such as disclosed in U.S. Pat. Nos. 3,318,212; 3,687,541; 3,697,171; 3,702,482; 3,844,653; and 3,844,654 or a belt such as disclosed in U.S. Pat. Nos. 3,671,118 and 3,697,170. The latter technique also suffers the disadvantages of increased cost, complexity and machine size and decreased reliability necessitated by the use of additional components intermediate transfer to a final support medium. Additionally, there is the probability of degradation in image quality of images transferred to the intermediate transfer member which must then be transferred to the final support medium. Furthermore, in some instances as disclosed in U.S. Pat. No. 3,318,212 the developed images are tackified by use of solvent vapors which are potentially flammable and which require the use of a consumable fluid which must be replaced periodically.
A further duplexing technique utilized in certain commercial electrophotographic machines and disclosed in prior art patents utilizes a single photoconductor wherein first fixed images developed sequentially on the first sides of a plurality of copy sheets by the electrophotographic process are collated in an intermediate tray and then sequentially transported back through the electrophotographic process to develop second fixed images on the second sides of the copy sheets, thus producing duplex copy sheets. This technique is disclosed in U.S. Pat. Nos. 3,615,129; 3,630,607; 3,645,615; British Pat. No. 1,450,842; U.S. Pat. Nos. 3,833,911; 3,856,295; 3,866,904; 3,917,256; 3,917,257 and 3,963,345. The latter three U.S. Patents also disclose copy sheet inverters in the exit path of the respectively disclosed apparatus in order to accommodate stacking or collation of simplex or duplex copy sheets after fixation of images thereto. The latter "two pass" process has several disadvantages. Since the first sides of all the copy sheets are developed before development of the second sides of the copy sheets, a duplex copy is not available for proofreading until all of the first sides and one set of second sides have been developed. In addition, the relatively long paper paths required in passing a copy sheet through the entire electrophotographic process twice greatly increases the possiblity of paper jams and other potential copy handling complications and also reduces copier efficiency and productivity. For example, when two or three page documents are copied, it may take longer for the first copy sheet to return for passage a second time through the copying process than for all of the other copy sheets to pass through the copying process the first time. Moreover, due to the long periods between forming and fixing images on the first and second sides of a copy sheet, the environmental conditions of image formation and the physical parameters of the copy sheet may change resulting in images of varying quality on opposite sides of a single sheet.
Still another duplex copying technique which may be considered especially relevant to the present invention involves fixing images to both sides of a copy sheet during a single pass through the disclosed electrophotographic processes. U.S. Pat. Nos. 3,506,347; 3,672,765; 3,869,202; and 3,947,270 disclose various embodiments of this technique. In the first patent just listed a first tackified image is formed on a transfer drum, the image is transferred to the first side of a copy sheet, the sheet is inverted whilst the first tackified image dries and becomes fixed on the copy sheet, a second tackified image is formed on the transfer drum and the copy sheet is fed back into contact with the drum to transfer the second image to the second side of the copy sheet which is then transported to an output tray. In the latter three patents electrophotographic apparatus is disclosed for making duplex copies wherein two images of an original are formed sequentially on an endless photoconductor, the images are developed and the first developed image is transferred to the first side of a copy sheet. The copy sheet is separated from the photoconductor, passed through a fuser to fuse or fix the first transferred image to the copy sheet which is then turned over and the opposite side of the copy sheet brought into contact with the second developed image on the photoconductor. The second image is then transferred to the second side of the copy sheet, the copy sheet separated from the photoconductor and the second image fused by means of a second fuser. The copy sheet is then transported to a copy receptacle. Although the apparatus disclosed in the latter three patents present a viable technique for forming duplex copy sheets, they do have certain disadvantages. Since the first image is fixed or fused before copy sheet turnaround, the use of two fusers is necessitated with attendant increase in cost, power and environmental heat. In addition, in the apparatus disclosed in U.S. Pat. No. 3,672,765 the photoconductor belt is fed around a roller spaced from the turnaround mechanism before transfer of the second image to a copy sheet. The resultant delay caused by copy sheet turnaround between transfer of the first and second images causes inefficient use of the photoconductive belt and slows down the photoconductive process. The use of solvent vapors to tackify the images as disclosed in U.S. Pat. No. 3,506,347 raises the problems of flammability and replenishing of the solvent. Moreover, use of a transfer drum unnecessarily complicates the electrophotographic process.
The prior art is also replete with disclosures of various configurations for turning around or inverting original or copy sheets or cards in various types of reproduction apparatus. Thus the inversion of duplex original document sheets for reproduction of both sides of the original in film reproduction apparatus is disclosed in U.S. Pat. Nos. 3,227,444; 3,408,140; 3,575,507 and in electrophotographic apparatus is disclosed in U.S. Pat. Nos. 3,561,865 and 3,675,999. Devices for inverting copy sheets or cards are also known in the art as exemplified in the disclosures of U.S. Pat. Nos. 2,901,246; 3,416,791; 3,523,687; and 3,848,868. Other sheet inverters are also disclosed in the prior art as exemplified by the disclosures of U.S. Pat. Nos. 2,904,334; 2,787,363; 3,008,707; 3,236,517; 3,389,906; 3,948,505 and French Pat. No. 2,219,013. None of the disclosures in the aforementioned patents disclose the concept of the present invention of forming unfixed images on opposite sides of a copy sheet before fixing of the images to the copy sheet.